I received it a few days ago and have been testing it (playing with it is a better term).

I have had less than adequate results so far. I can not get the input and output levels correct without resulting in clipping or high distortion. I am beginning to think it is my Windows 7 Pro 64Bit drivers from M-Audio that is causing my problems. I am currently looking for a laptop with XP Pro to borrow from one of my friends.

One of my other concerns using this card on my laptop is that the laptop uses a miniature 4 pin firewire port. This requires that I use the included power adapter.

ASUS make a broad range of no nonsense, well designed, audio cards. The top of the range essence ST/STX have fantastic performance as can be seen from their reviews over at stereophile. I have the ST and can only confirm what stereophile concluded.

Now these are reasonably expensive and the other ASUS cards basically use the same interface hardware but with cheaper conveters/analogue stages. There's no shortage of reviews when it comes to finding out how their cards perform in a loop back test so you could go hunting for which of their cards will give you the performance you are after. Note that most of them perform extremely well, some though aren't so impressive, but then you do get what you pay for.

Besides ASUS you've got RME, some of their products are supposed to be excellent. SoNic_real_one has had good results with an E-MU 1820 but those seem a bit harder to find. The ESI Juli@ card is the way to go if you want to DIY in you own ADC/DAC units as they are modular and come with expansion connectors that will give you easy access to power and clock lines.

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That looks like very decent performance. The thd at 0.0004% makes the card suitable for a lot of things. Try firing STEPS and doing a 20-20k measurement, you can also do a measurement of amplitude vs distortion too.

The noise isn't amazingly low but it's low enough. The fact the adc/dac only seems to show second and third harmonics is good also, but perhaps if the noise floor was lower they might be apparent. Of course the Noise + thd value is degraded simply because the signal level has been reduced for the best thd value.

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What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz!

On many of these sound cards, the generator (DAC) is not as good as the ADC. Usually performance can be improved with a good external low distortion sine generator. HP339A, Tek SG505, etc.
Also note there will be high frequency, out of audio band noise (~MHz) from the sound card source as well. You'll easily see it on a 100MHz or greater scope

Also the software used to generate sine wave can have an influence. I have not played with this for a while, so can't say how ARTA does in this area, but it looks pretty good. There was an older thread here with measurements back around 2009 see my comments and measurements starting here

My concern is setting this up consistently each time I may need to take amplifier measurements. I cannot achieve the same results each time I start the program. It is finicky.

I am still not sure how to determine the noise floor of the E-MU with the sample screen shots I provided above.

What is the noise floor measure in? dBFS, dBV, dBu.

@ BFNY - I have this forum post in my favorites. I keep picking away at all the information there. Thanks. I tried your suggestions. I did not see any difference in THD or THD + N by changing the sample frequency. It makes since what you have said, and I will keep playing with these setup suggestions.

I have an HP 239 oscillator (same built in generator as the HP339). This is a great piece of equipment. I will try this tomorrow for an input.

I sure do like analog test equipment. I like to connect a few cables, push a few buttons, turn a few knobs and the read the results. I just can not afford a good working HP Spectrum Analyzer at the moment.

From those two graphs I would say that the noise would be at -103.8dB.

Apart from consistency one thing that is important in high resolution measurements is making sure that everything is connected in a way that will keep ground loops at bay. This shouldn't be too much of an issue with a laptop and external USB interface though as the whole thing can be battery operated and thus floating and create no loops.

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What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz!

@5th - Thanks. That is exactly what I thought the noise floor was. So I was correct in my set up. First establish a decent spectrum scan and then turn off the generator. What ever the RMS value is in the lower left is the noise floor. Easy.

What units of measurement is noise normally referenced?

Here is my STEPS scan. I am still working on the "amplitude vs distortion" scan. But I am close.

That's just the frequency response (which looks good btw). Run the sweep again but then increase the vertical scale so that 0dB is near the top and then say something like -130dB is at the bottom, then you'll get the harmonics plotted and we can get a good measurement of distortion vs frequency. You can alternatively click on the % button and it'll open a new window where distortion is shown as a %. I prefer the % window generally but the decibel scale csn also be useful.

Amplitude vs distortion is under the record tab, which I'm sure you've found. You basically set what frequency to run the test at (if thd is flat vs frequency from the previous measurement then the frequency you use here isn't important, 1k is nice to use though) then off you go. You can pick how many STEPS () it will take when doing the measurement and the amplitude it will start and end at. Now depending on how you've got the input sensitivity configured in the setup, this could be any value/voltage. Strictly speaking the value itself isn't important all you want it to do is stop at the maximum level that the card can output and start a reasonable way down from this. What this does, is show you any trends in the system that you should be aware of, like sudden rises in distortion etc that you'd want to avoid when making measurements.

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What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz!

Noise is normally expressed as normalized per unit of frequency. If you look at op-amp and other discrete transistors, you'll see specs like nV/sqrt (Hz).

In simple terms, noise is a function of bandwidth. The more bandwidth, the more noise.
When you use a measurement tool like a frequency limited analog voltmeter after a notch filter (the typical analog THD+N analyzer i.e. HP339A) the bandwidth determines the total noise. If you examine closely audio specs on noise, this sort of thing is well documented. There are specs that define filters to band limit frequencies for many noise measurements.

When you use a measurement tool like a parallel filter frequency analyzer, things are different. You really need to be involved in the industry or study it closely to understand this. With a parallel filter frequency analyzer, the noise you measure is related to the bandwidth of the many parallel filters. The more narrow the filters, the smaller the noise floor you'll see on the frequency display. The filters can be constant bandwidth (FFT) or constant percentage bandwidth (octave, 1/3, 1/12 octave, etc)
In general, that's one of the reasons why I recommend using large sample block sizes, as above. It makes the FFT "filters" extremely narrow, and allows you to concentrate better on the distortion numbers.

Started doing this stuff with FFT's in '81 using a Nicolet Mini Ubiquitous 444 FFT box. Still doing it at GHz frequencies.
Bob